packer-cn/vendor/github.com/klauspost/compress/flate/snappy.go

97 lines
2.7 KiB
Go

// Copyright 2011 The Snappy-Go Authors. All rights reserved.
// Modified for deflate by Klaus Post (c) 2015.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package flate
// We limit how far copy back-references can go, the same as the C++ code.
const maxOffset = 1 << 15
// emitLiteral writes a literal chunk and returns the number of bytes written.
func emitLiteral(dst *tokens, lit []byte) {
ol := dst.n
for i, v := range lit {
dst.tokens[i+ol] = token(v)
}
dst.n += len(lit)
}
// emitCopy writes a copy chunk and returns the number of bytes written.
func emitCopy(dst *tokens, offset, length int) {
dst.tokens[dst.n] = matchToken(uint32(length-3), uint32(offset-minOffsetSize))
dst.n++
}
// snappyEncode uses Snappy-like compression, but stores as Huffman
// blocks.
func snappyEncode(dst *tokens, src []byte) {
// Return early if src is short.
if len(src) <= 4 {
if len(src) != 0 {
emitLiteral(dst, src)
}
return
}
// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
const maxTableSize = 1 << 14
shift, tableSize := uint(32-8), 1<<8
for tableSize < maxTableSize && tableSize < len(src) {
shift--
tableSize *= 2
}
var table [maxTableSize]int
// Iterate over the source bytes.
var (
s int // The iterator position.
t int // The last position with the same hash as s.
lit int // The start position of any pending literal bytes.
)
for s+3 < len(src) {
// Update the hash table.
b0, b1, b2, b3 := src[s], src[s+1], src[s+2], src[s+3]
h := uint32(b0) | uint32(b1)<<8 | uint32(b2)<<16 | uint32(b3)<<24
p := &table[(h*0x1e35a7bd)>>shift]
// We need to to store values in [-1, inf) in table. To save
// some initialization time, (re)use the table's zero value
// and shift the values against this zero: add 1 on writes,
// subtract 1 on reads.
t, *p = *p-1, s+1
// If t is invalid or src[s:s+4] differs from src[t:t+4], accumulate a literal byte.
if t < 0 || s-t >= maxOffset || b0 != src[t] || b1 != src[t+1] || b2 != src[t+2] || b3 != src[t+3] {
// Skip 1 byte for 16 consecutive missed.
s += 1 + ((s - lit) >> 4)
continue
}
// Otherwise, we have a match. First, emit any pending literal bytes.
if lit != s {
emitLiteral(dst, src[lit:s])
}
// Extend the match to be as long as possible.
s0 := s
s1 := s + maxMatchLength
if s1 > len(src) {
s1 = len(src)
}
s, t = s+4, t+4
for s < s1 && src[s] == src[t] {
s++
t++
}
// Emit the copied bytes.
// inlined: emitCopy(dst, s-t, s-s0)
dst.tokens[dst.n] = matchToken(uint32(s-s0-3), uint32(s-t-minOffsetSize))
dst.n++
lit = s
}
// Emit any final pending literal bytes and return.
if lit != len(src) {
emitLiteral(dst, src[lit:])
}
}